In grinding cylindrical (e.g. spur or helical) gears with a threaded grinding wheel, it is customary to move or “shift” the workpiece relative to the grinding wheel in the direction of the grinding wheel axis of rotation. Shifting may occur during gear grinding (known as continuous shifting), that is, shifting may occur simultaneously with the stroke of the grinding wheel along the length of the gear teeth thus resulting in a diagonal movement between the grinding wheel and workpiece.
Shifting in the direction of the grinding wheel axis may also occur between strokes (such as between roughing and finishing grinding strokes) or after the final grinding stroke (e.g. after the finish stroke). This type of shifting, known as incremental shifting, is usually intended to position the workpiece at another location on the grinding wheel such as for finish grinding, or to reposition the grinding wheel from a finish grind position of one workpiece to a start position for the next workpiece. Incremental shifting may take place between any or all strokes in either direction of the grinding wheel axis.
Shifting distributes wear evenly along the length of a grinding wheel and brings fresh portions of the grinding wheel into contact with a workpiece which makes it possible to remove a maximum amount of metal in a short period of time without wheel loading or gear burning. Wheel loading is understood to be a condition where metal particles adhere to a grinding wheel thereby filling up pores in the grinding wheel which causes an increased load or power draw on the grinding wheel. Gear burning is understood to be a condition where enough heat is generated during the grinding process to cause the metal of the gear tooth flanks to temper. Severe burning can be seen visually, however in most instances, a special detection process must be used to determine if a gear has been burned. Wheel loading can also cause gear burning.
In shifting from a beginning location to an end location along the axis of the grinding wheel, a corresponding amount of grinding thread surface area is utilized. The amount of thread surface area is dependent upon the amount of continuous shift (CS), the diameter (or radius) of the grinding wheel and the lead angle (λ) of the grinding thread.
However, as a grinding wheel is used, the grinding thread becomes worn and, therefore, the grinding wheel must be periodically dressed to restore the grinding thread to its desired form. With each dressing, the diameter of the grinding wheel is reduced. As the diameter of a grinding wheel is reduced, the amount of thread surface area utilized in shifting is likewise reduced. Thus, the grinding process is not constant and the reduced amount of thread surface area per shift increases the likelihood of wheel loading and/or gear burning, which can affect the quality of the gear being produced.